Plato later in the Republic, identifies three elements of the human soul, or psyche. Different elements will come to prominence under differing political arrangements. There exists, says Plato, desire or appetite on one extreme and reason or rationality on the other. Inbetween is thymos, a vague category often translated as 'spirit' or 'will'. A concept of justice expressed by Thrasymachus as that which is in the interest of the stronger party owes its origin to the dominance of desire. Reason however Plato considers the superior element of the soul.
An understanding of justice then requires not instinctual 'political' (in the most basic sense) judgements, changeable and merely contextual relevant, but a rational and reasoned philosophical base. Justice requires knowledge. "Isn't it characteristic of the honourable to subjugate the bestial parts of human nature to the human part (we might, perhaps, rather call it the divine part) ? " Plato asks for philosophy to bring humanity towards another ideal which cynical modern political theory has long abandoned, that of the summun bonnum, the 'highest good, realised in the perfect state.
Plato acknowledges human imperfection, "… sometimes a person's nature has some sort of weakness in its best aspect, which makes that aspect able only to minister to the broods of desires within him, and not to rule them," but it is then the purpose of philosopher – rulers to educate and rule in a way which "impress[es] upon him [the man ruled by desire] from outside, so that as far as possible we may all be alike and all be friends, since we are all steered by the same helmsman.
" The 'ship of state' (the metaphor to which Plato here is referring) must be sailed by philosophers, those possessed with knowledge of the forms and not seduced through desire by mere appearances, which is for the benefit of all. Philosophy plays a large part in the discussion of justice in the Republic because this, the pursuit of philosophy, is the basis of the platonic project.
This application of idealism to practical situations has dominated political thinking from Plato until the late nineteenth century; modern political philosophy has been summarised succinctly as 'footnotes to Plato. ' Plato's ideas had practical ends – he established his Academy to train philosopher rulers and realise the proposals in the Republic, but it was theory which determined practice and without solid theory, practical failure (such as the defeat of his native Athens at the hands of Sparta) would surely follow.